The mammalian olfactory system is increasingly recognized as an attractive model system for studying the formation of neural circuits during development. An olfactory sensory neuron (OSN) expresses one the of approximately 1000 intact odorant receptor (OR) genes in the mouse genome. Cell bodies of OSNs expressing a given OR gene are dispersed in one of four zones within the olfactory epithelium, and their axons converge onto a few of the approximately 1800 glomeruli in the olfactory bulb of the mouse. A mapping problem is thus posed: 1000 populations of OSNs, each expressing a distinct OR, must be sorted onto approximately 1800 glomeruli. What defines the site in the bulb to which axons of OSNs expressing a given OR or responding to given odorants converge? Progress in our understanding of the parameters defining glomerular position has been slow. No systematic studies of the glomerular array have been undertaken, and the literature is fragmented across species, odorants, ORs and glomeruli. A productive approach to study glomerular convergence has been targeted mutagenesis of OR genes to express axonal markers in OSNs expressing a given OR gene. Such experiments have revealed that the OR is a critical determinant of axonal convergence to glomeruli. Here we propose a coordinated, systematic approach focusing on the mouse and based on meaningful subsets of ORs and glomeruli. The approach is an integrated combination of molecular biology and genetics, physiology, and anatomy. Our hypothesis is that parameters defining a neighborhood in the glomerular array of the mouse olfactory bulb are: OR sequence features, odorant responsiveness and onset and zone of OR expression. Specific Aim 1: How are neighboring glomeruli defined in terms of odorant receptor sequence and odorant responsiveness? Specific Aim 2: What is the glomerular organization of two families of related OR genes at a known genomic locus? Specific Aim 3: How are time and zone of OR expression related to glomerular position?